Photoresist patterning is a key step in the formation of integrated circuits in semiconductor devices. A photoresist layer is typically spin coated on a substrate and patternwise exposed by employing an exposure tool and a mask that contains a device pattern. Radiation is transmitted through transparent regions of the mask to selectively expose portions of the photoresist layer which are later developed in a media such as aqueous base to produce a photoresist pattern on the substrate. Each technology generation or node in the microelectronics industry is associated with a particular minimum feature size in the photoresist pattern. As technology advances have been continuous in recent years, the minimum feature size requirement has rapidly shifted from 250 nm to 180 nm and then to 130 nm. New products are now being developed for a 100 nm technology node.
Some of the more common features that are printed in photoresist layers are contact holes and line/space arrays in which line and space widths can occur in various sizes. The patterned photoresist generally functions as a mask for a subsequent etch step in which the device pattern is transferred into the substrate. Optionally, it can be employed as a mask for an ion implant process. The minimum resolution that can be achieved in a printed pattern is defined by the equation R=kλ/NA where R is the minimum feature size that can be resolved, k is a constant, λ is the exposure wavelength, and NA is the numerical aperture of the exposure tool. While exposure tools having mercury lamps that emit g-line (436 nm) or i-line (365 nm) radiation have been widely used in the industry, the trend in newer technologies is to move to shorter wavelengths such as 248 nm from KrF excimer lasers or 193 nm from ArF excimer lasers to achieve smaller feature sizes. In the near future, 157 nm radiation from F2 lasers and 13 to 14 nm wavelengths from extreme ultraviolet radiation (EUV) sources will be available.
Commercial photosensitive compositions are available in two general types that are referred to as positive tone and negative tone formulations. In positive tone photoresist, exposed regions become soluble in a developer solution that is typically an aqueous base. Unexposed regions in the film remain insoluble in the developer and remain on the substrate. For negative tone photoresists, exposed regions become insoluble in a developer while the unexposed regions remain soluble and are washed away. The thickness of the photosensitive film can vary from about 0.2 microns to several microns. As a general rule, the film thickness is about 3 or 4 times the size of the minimum space width or line width. Therefore, to print a 100 nm contact hole, a 300 to 400 nm thick film is desirable in order to have a process latitude that is manufacturable.
Most state of the art positive and negative tone photoresists operate by a chemical amplification mechanism in which a photosensitive component absorbs energy from the exposing radiation and generates a strong acid. One acid molecule is capable of removing many polymer protecting groups in a positive tone mechanism or initiating several crosslinking reactions in a negative tone composition. A post-exposure bake is usually required to drive the reactions to completion within a few minutes so that the process is compatible with a high throughput manufacturing scheme. Chemically amplified (CA) photoresists are especially compatible with Deep UV radiation from a 248 nm excimer laser or from a Xe/Hg lamp that is filtered to transmit primarily 240 nm to 260 nm wavelengths. CA photoresists are also useful with sub-240 nm wavelengths.
The negative tone imaging process can involve a crosslinking mechanism or a polarity change to render the exposed regions insoluble in developer. Crosslinking occurs when a photo generated acid catalyzes bond formation between two polymer chains or between a polymer and an additive containing reactive groups. Depending on the molecular weight (MW) of the original polymers, a few crosslinks are all that might be needed to convert a soluble polymer into an insoluble network of polymers. This solubility difference is the basis for forming a pattern in an exposed negative tone film. Optionally, a solubility difference can be achieved if the photo generated acid induces a change within the polymer. For example, a polar polymer containing hydroxy groups can react with acid in exposed regions to lose its polar functionality by rearranging or by cleaving into two or more parts and thereby become insoluble in developer solution.
Traditionally, photoresists have been formulated in organic solvents, but recently water based formulations that are more environmentally compatible have been developed. U.S. Pat. No. 5,017,461 describes a water soluble negative tone composition based on a polyvinyl alcohol (PVA) and an acid generator that is a diazonium salt. An hydroxyl group on the polymer reacts with the diazonium salt to form an ether and liberate nitrogen and HCl. When the film is heated, HCl induces the polymer to lose a molecule of water and form an alkene that is insoluble in water developer.
Another water soluble negative tone photoresist that does not rely on a crosslinking mechanism is provided in U.S. Pat. No. 5,998,092. A photoacid reacts with an acetal group on a polymer side chain to produce a B-keto acid that loses CO2 to form a polymer which is insoluble in aqueous base developer. This composition is especially useful in avoiding swelling in aqueous developer.
Individual components of negative tone photoresists have been developed that possess water solubility as an added property. For example, a water soluble sugar is claimed as an improved crosslinker in related U.S. Pat. Nos. 5,532,113 and 5,536,616. This crosslinker is used in combination with a p-hydroxystyrene polymer and a triphenylsulfonium salt that are not soluble in water and have an optical absorbance that is most suitable for 248 nm exposures. The pattern is developed in aqueous base. In U.S. Pat. No. 5,648,196, a water soluble photoacid generator (PAG) is described and is formulated with a p-hydroxystyrene polymer and a water soluble sugar. Either water or aqueous base developer is acceptable. The PAG is preferably a dimethylarylsulfonium salt wherein the aryl group has one or more hydroxy substituents.
A good lithography process, whether it involves a negative or positive tone material, is characterized by its process latitude. An acceptable process window for manufacturing depends on the minimum feature size but generally a depth of focus (DOF) of at least 0.5 to 1 micron and an exposure latitude of greater than 10% are desired for sub-micron feature sizes. In other words, the best focus setting may drift from 0.5 to 1 micron because of topography or reflectivity variations on the substrate and the dose delivered by the exposure tool may vary by up to 10% but the resulting feature size can still be maintained within a +/−10% tolerance of the intended linewidth or space width.
The patterning of contact holes for current and next generation devices is a difficult challenge for lithography engineers. A major concern is the inadequate DOF for small contact holes. Although exposure tools with higher NA are expected to improve the exposure latitude and resolution capability, DOF will be reduced according to the equation, DOF=λ/(NA)2. Various process techniques such as attenuated phase shift masks (Att. PSM), high transmission Att. PSM, and scattering bar (SB) technology have been developed and applied to improve the process window for printing contact holes by optical lithography. Unfortunately, these methods induce a sidelobe pattern in the photoresist and surface damage that will cause damage to the device during a subsequent etch transfer step.
Sidelobe formation is more likely to occur under conditions when a photoresist is slightly overexposed or if the photoresist has a tendency to lose film thickness in unexposed areas during development. It should be noted that “unexposed” photoresist regions do receive a small dose of stray light because of an imperfect aerial image from the mask. As a result, there is a small film thickness loss in unexposed regions after pattern development which can vary from a few Angstroms to several hundred Angstroms. Since the exposure latitude is restricted because higher doses tend to cause sidelobe formation, the process latitude is further reduced over the loss from a higher NA or lower λ as predicted in the equation, DOF=λ/(NA)2.
Yung-Tin Chen, Ya-Chi Wang, and Ron Chu reported in “Optimization of Attenuated Phase Shift Mask for Contact Hole Printing” presented in SPIE Conference on Optical Microlithography XII, March, 1999, pages 812-820, that a pretreatment of an i-line (365 nm photosensitive) resist which removes part of the film thickness before exposure can extend the exposure latitude before sidelobe formation occurs. They also mention that the effect of exposure tool NA and sigma (partial coherence) have a different effect on the onset of sidelobes for isolated contact holes and dense contact holes.
A cross-sectional view of a prior art process in which a side lobe is formed and transferred into a substrate is shown in FIGS. 1a-1d. A substrate 10 is provided that typically contains active and passive devices in a substructure not shown in order to simply the drawing. Optionally, an anti-reflective coating (not shown) is formed on substrate 10 in order to control reflectivity during photoresist patterning. Then a positive tone photoresist is spin coated and baked to form photoresist layer 11. A reticle 12 having a patterned opaque coating 13 and a transparent substrate 14 is positioned between the radiation source (not shown) and the substrate 10. A single wavelength or broad band of wavelengths 15 is emitted by the source and passes through transparent substrate 14 not covered by opaque coating 13 on reticle 12. Reticle 12 can be an Att. PSM, an alternating phase shift mask (Alt. PSM) or a binary mask comprised of only chrome on quartz.
The exposed photoresist 11 is post exposed baked and then developed in an aqueous base solution to form the pattern shown in FIG. 1b. Because of a combination of conditions including illumination settings, type of mask, contact hole design, and other factors, a sidelobe 18 is formed between contact holes 16 and 17. The pattern is etch transferred into substrate 10 as shown in FIG. 1c. Although photoresist 11 is an etch mask, its thickness is reduced during the etch process. Then photoresist 11 is stripped to leave a pattern in substrate 10 that includes the desired contact holes 16a and 17a. However, since sidelobe 18 extended deep into photoresist 11, the film thickness below sidelobe 18 was not sufficient to prevent the etch from penetrating into substrate 10 and forming an unwanted divot 19 as depicted in FIG. 1d that will cause a degradation in device performance. Even if the sidelobe in FIG. 1c is found by optical inspection of the substrate and the etch step is not allowed to proceed, the substrate must be reworked by stripping the photoresist and repeating the patterning step. These unplanned steps drive a higher cost for the device and result in a disruption in normal process flow through the manufacturing line. Attempts to minimize the sidelobe will only result in a smaller process window that will lead to a higher amount of rework in succeeding batches of substrates.
Besides the single layer photoresist shown in FIGS. 1a-1d, a bilayer or trilayer patterning scheme is possible. In a bilayer design, a thin photosensitive composition is coated on a thicker underlayer that is not photosensitive. The pattern that is formed in the top layer is etch transferred through the bottom layer and then into the substrate. A bilayer method is especially effective when the substrate has topography or if a single layer resist does not provide enough etch resistance for an etch transfer process. With a trilayer method, a third layer is typically inserted between the top and bottom layer of a bilayer scheme. The extra layer is not photosensitive and normally provides added etch resistance. A top photosensitive layer is involved in both bilayer and trilayer schemes and is subject to the same sidelobe formation tendency as in a single layer process because the illumination conditions, mask, and contact hole design that contribute to sidelobe formation in a single layer photoresist have not changed.
Therefore, a method which can solve the sidelobe problem without reworking the substrate or reducing process window is needed. The method should accommodate a wide variety of exposure wavelengths as well as different types of masks and photoresist materials. Ideally, a method is needed that is compatible with single layer, bilayer or trilayer patterning schemes.